The present invention relates to a method for anticorrosion treatment of the outer surface of a heat exchange tube made of an aluminum extrusion (hereinafter may be referred to as an “aluminum extruded heat exchange tube,” and to a method for producing a heat exchanger. More particularly, the present invention relates to a method for anticorrosion treatment of the outer surface of an aluminum extruded heat exchange tube for use in car air conditioner condensers, evaporators, and heater cores, and radiators which are installed in vehicles such as automobiles, and to a method for producing a heat exchanger having an aluminum extruded heat exchange tube whose outer surface has been subjected to anticorrosion treatment.
As used herein, the term “aluminum” refers to elemental aluminum as well as to aluminum alloys. Materials represented by an elemental symbol refer to pure materials of the corresponding element.
A typical condenser widely used in car air conditioners includes a pair of header tanks formed of aluminum, a plurality of flat heat exchange tubes formed of aluminum extrusion, corrugated fins formed of aluminum, and side plates formed of aluminum. The header tanks are disposed such that their longitudinal direction coincides with a vertical direction and they are spaced apart from each other. The heat exchange tubes are disposed between the two header tanks such that their width direction coincides with an air-passing direction, and they are spaced apart from each other in the vertical direction. Opposite end portions of the heat exchange tubes are brazed to the corresponding header tanks. Each of the fins is disposed between adjacent heat exchange tubes, on the upper side of the upper-end heat exchange tube, or on the lower side of the lower-end heat exchange tube. The fins are brazed to the corresponding heat exchange tubes. The side plates are disposed on the upper side of the upper-end fin and on the lower side of the lower-end fin, and are brazed to these fins. Such a condenser is produced by simultaneously bonding header tanks (including a member for forming the header tanks), heat exchange tubes, and fins, through brazing.
Meanwhile, since condensers for car air conditioners are used under corrosive circumstances, pit corrosion of walls of heat exchange tubes, which would otherwise occur within a relatively short period, must be prevented, so as to prevent leakage of a refrigerant from the heat exchange tubes.
Conventionally, there has been proposed a heat exchanger production method for preventing occurrence, within a relatively short period, of pit corrosion of walls of heat exchange tubes of a condenser for car air conditioners. The production method includes providing aluminum extruded heat exchange tubes each being formed of an alloy comprising Mn 0.15 mass % and Cu 0.4 mass %, the balance being Al, and unavoidable impurities, and corrugated fins formed from a brazing sheet composed of an aluminum core which is made of Mn 1.2 mass %, Cu 0.15 mass %, and Zn 2.5 mass %, the balance being Al, and unavoidable impurities, and a cladding which is made of an aluminum braze of AA4343 and covers both surfaces of the core; applying a dispersion of a flux powder and a Zn powder in a binder onto the outer surface of the heat exchange tube, and vaporizing a liquid component of the dispersion, to thereby deposit the Zn powder and the flux powder on the outer surface of the heat exchange tube, such that the Zn powder deposition amount is adjusted to 2 to 4 g/m2, the flux powder deposition amount is adjusted to 15 g/m2 or less, and the ratio of the flux powder deposition amount to the Zn powder deposition amount (flux powder deposition amount/Zn powder deposition amount) is adjusted to 1 or less; and
combining and heating the heat exchange tubes and the fins, to thereby braze the heat exchange tubes to the fins by use of the flux powder deposited on the outer surface of each heat exchange tube and the cladding of each fin, and melting the Zn powder deposited on the outer surface of each heat exchange tube, and then diffusing Zn to an outer surface portion of the heat exchange tube, to thereby form a Zn diffusion layer in the outer surface portion of the heat exchange tube (see Japanese Patent Application Laid-Open (kokai) No. Hei 11-216592).
In the method disclosed in the patent publication, pit corrosion, which would otherwise occur within a relatively short period in walls of heat exchange tubes, is prevented through sacrificial corrosion of the Zn diffusion layer. However, the ratio of the thickness of the Zn diffusion layer to the wall thickness of each heat exchange tube increases excessively, due to the composition of the alloy forming the heat exchange tubes, a ratio of the amount of flux powder deposited on the outer surface of each heat exchange tube to the amount of Zn powder deposited on the outer surface (flux powder deposition amount/Zn powder deposition amount) being 1 or less, a large particle size of Zn powder (not described in detail), and other factors. Thus, when the wall thickness of the heat exchange tube is small; e.g., 200 μm or less, through holes may be formed in walls of the heat exchange tubes within a relatively short period of time.
Japanese Patent Application Laid-Open (kokai) No. Hei 11-216592 does not specifically disclose the wall thickness of the employed aluminum extruded heat exchange tube. However, Japanese Patent Application Laid-Open (kokai) No. 2006-2212 (paragraph 0039) discloses the wall thickness of the extruded heat exchange tube having the outer surface onto which a Zn-containing flux is applied. The extruded heat exchange tube, which is used in a manner similar to that of the heat exchange tube disclosed in Japanese Patent Application Laid-Open (kokai) No. HEI 11-216592, has a wall thickness of about 400 μm.